Unfortunately, we cannot do much to speed up `SelectFolder` method. Can you please send us communication log, so we can see possible issue. It can be done like this:
imap.LogWriter = new Rebex.FileLogWriter("your.log.path", Rebex.LogLevel.Debug);
If you don't need to modify mails in your folders, you can try read-only select like this:
Unfortunately, client can search for mails in selected folder only. There is no way to search for mails in more folders.